The inventive concepts disclosed herein relate generally to the field of alignment of a heads up display (HUD) in an aircraft.
A HUD may be used by an operator in an aircraft to allow the operator to have a view of the outside world with superimposed information, such as symbols indicating nose direction of the aircraft, altitude, and touch down zone location where the aircraft is intended to land. It is important that the HUD be sufficiently aligned with the aircraft's three angular components, yaw, pitch, and roll, so that the superimposed information, such as the touch down zone location, is displayed in an accurate location on the HUD. Such alignment is called boresighting. A typical requirement for HUD accuracy is 3 mRad with respect to the outside world.
The alignment of the HUD is affected by the aircraft boresight itself, HUD hardpoint installation accuracy, HUD electronic boresighting, rigidity of the aircraft structure, and the location of an inertial reference unit (IRU) attached to the aircraft. The aircraft boresight itself is defined by the IRU attached to the aircraft, where the IRU provides the aircraft's three angular components, yaw, pitch, and roll.
Typical HUD hardpoint installation is performed using a target board and alignment tool while the aircraft is on the ground. The alignment tool is arranged in the aircraft cockpit, where the HUD is also arranged, while the target board is located outside of the cockpit. The alignment tool is then adjusted to align with the target board. Due to the stringent HUD accuracy requirements of about 3 mRad, the hardpoint installation requires complicated tooling to characterize the hardpoint installation, and is a time consuming process that takes up manufacturing floor space and increases cost.
HUD electronic boresighting provides for deviations from the hardpoint installation for a HUD in the cockpit. The deviations are stored in a personality module, which electronically corrects for the deviations from hard point installation for a particular HUD.
Rigidity of the aircraft and the location of the aircraft IRU on the aircraft must also be taken into account when boresighting the HUD. Typically, the aircraft IRU is disposed in a location near the wings of the aircraft in the wing box. The structure of the aircraft, however, is subject to distortion during flight, which must be compensated for in order to have accurate HUD boresighting. For example, during flight the aircraft may be subject to “nose droop,” where the nose of the aircraft sags downward. In this case, the boresight of the aircraft as defined by the aircraft IRU in the wing box becomes misaligned with the HUD boresight, which is in the region near the nose of the plane, which suffers from nose droop.
The HUD boresighting may have a predetermined passive nose droop compensation. Such predetermined nose droop compensation, however, corrects for nose droop for one flight condition, and may cause the HUD to become misaligned in other conditions.